The process of defecation requires coordinated action of the smooth and striated muscle layers in the rectum and anus, as well as surrounding structures such as the pelvic floor muscles. During the mass movement of feces produced by high-amplitude propagating contractions, the rectum fills with fecal material. Expulsion of this material from the body is controlled by the internal and external anal sphincters, which contribute approximately 70% to 80% and 20% to 30% of anal tone at rest, respectively. Filling of the rectum causes relaxation of the internal anal sphincter via release of vasoactive intestinal polypeptide and generation of nitric oxide. Relaxation of the inner sphincter permits the anal sampling mechanism, which can distinguish whether the rectal contents are solid, liquid, or gaseous in nature. After toilet training, sensory nerve endings in the anal mucosa then generate reflexes that initiate appropriate activity of the external sphincter to either retain the rectal contents or permit voluntary expulsion (e.g., of flatus). If defecation is not convenient, the external sphincter contracts to prevent the loss of stool. Then with time the rectum accommodates to its new volume, the internal anal sphincter contracts again, and the external anal sphincter relaxes (
Fig. 31.6
When defecation is desired, on the other hand, adoption of a sitting or squatting position alters the relative orientation of the intestine and surrounding muscular structures by straightening the path for exit of either solid or liquid feces. Relaxation of the puborectalis muscle likewise increases the rectoanal angle. After voluntary relaxation of the external anal sphincter, rectal contractions move the fecal material out of the body, sometimes followed by additional mass movements of feces from more proximal segments of the colon (
Fig. 31.7 ). Evacuation is assisted by simultaneous contraction of muscles that increase abdominal pressure, such as the diaphragm. Voluntary expulsion of flatus, on the other hand, involves a similar sequence of events, except that there is no relaxation of the puborectalis muscle. This permits flatus to be squeezed past the acute rectoanal angle while retaining fecal material.
Cooperative activity of the external anal sphincter, puborectalis muscle, and sensory nerve endings in the anal canal is required to delay defecation until it is appropriate, even if the rectum is acutely distended with stool or intraabdominal pressure rises sharply. This explains why incontinence can develop in individuals in whom the integrity of such structures has been compromised, such as after trauma, surgical or obstetrical injuries, prolapse of the rectum, or neuropathic diseases such as long-standing diabetes. Surgical intervention may be necessary to correct muscle abnormalities in patients with the distressing condition of fecal incontinence, although many can be helped to increase external anal sphincter tone with the use of biofeedback exercises.
•Fig. 31.6 Responsesoftheinternalandexternalanalsphincterstoprolongeddistentionoftherectum.Notethattheresponsesofthesphinctersaretransientbecauseofaccommodation.(RedrawnfromShusterMMetal.Bull Johns Hopkins Hosp 1965;116:79.) •Fig. 31.7 Motilityoftherectumandanalsphinctersinresponsetorectalfillingandduringdefecation.Notethatfillingoftherectumcausesaninitialdecreaseininternalsphinctertonethatiscounterbalancedbycontractionoftheexternalsphincter.Theinternalsphincterthenaccommodatestothenewrectalvolume,therebyallowingtheexternalsphinctertorelax.Finally,defecationoccurswhentheexternalanalsphincterisrelaxedvoluntarily.(DatafromChangEBetal.Gastrointestinal, Hepatobiliary and Nutritional Physiology. Philadelphia:Lippincott-Raven;1996.) 1.